Final answer:
The M protein in S. pyogenes aids its invasiveness by impeding phagocytosis, as it alters the bacterial surface and blocks the binding of complement molecules, thus evading one of the immune system's primary defense mechanisms.
Step-by-step explanation:
Among the virulence factors produced by bacterial pathogens, some are specifically adept at evading the immune system. For instance, S. pyogenes, which is a species of Streptococcus, contains M protein in its fimbriae. M protein significantly contributes to the bacterium's ability to cause disease by impeding phagocytosis. This is accomplished by altering the surface structure of Streptococcus and inhibiting the binding of complement molecules that assist phagocytes in the consumption of bacterial pathogens. The complement system plays a critical role in the immune response by marking pathogens for destruction, and evading this system allows S. pyogenes to bypass one of the body's primary defenses against infection. Therefore, the correct answer to the student's question is D. Impedes phagocytosis.